There has conventionally been known a wide-band optical amplifier which can amplify incident light whether the wavelength band of the incident light is the 1.55 μm band (C-band: 1.53 μm-1.56 μm) or the 1.58 μm band (L-band: 1.57 μm-1.61 μm) (refer to a patent document 1 (Japanese Laid-Open Patent Publication (Kokai) No. 2001-358389 (Abstract)), for example).
The wide-band optical amplifier of this type amplifies the incident light by passing the incident light through a first Er (Erbium)-doped optical fiber if the wavelength band of the incident light is the C-band. The wide-band optical amplifier amplifies the incident light by passing the incident light through the first Er-doped optical fiber, and then a second Er-doped optical fiber if the wavelength band of the incident light is the L-band. The light which has been amplified is output as amplified light from the wide-band optical amplifier.
On this occasion, in order to reduce a noise of the amplified light, there may be further provided an Er-doped optical fiber for low-noise amplification on a prior stage of the first Er-doped optical fiber. If the gain of the Er-doped optical fiber for low-noise amplification is increased, it is possible to reduce an influence of the noise due to the first Er-doped optical fiber.
However, if the lengths of the first Er-doped optical fiber and the Er-doped optical fiber for low-noise amplification are determined to reduce the noise of the amplified light within the C-band output by the first Er-doped optical fiber, the gain of the output of the first Er-doped optical fiber decreases. As a result, an influence of a noise due to the second Er-doped optical fiber increases, and it is thus not possible to reduce the noise of the amplified light within the L-band.
On the other hand, if the lengths of the first Er-doped optical fiber and the Er-doped optical fiber for low-noise amplification are extended to reduce the noise of the amplified light within the L-band, it is not possible to reduce the noise of the amplified light within the C-band.
In this way, it is difficult to reduce both the noises of the amplified light within the C-band and the amplified light within the L-band.
It is thus an object of the present invention to provide an optical amplifier which can be applied to different wavelength bands, and outputs amplified light with a reduced noise within either of the wavelength bands.